1. Field of the Invention
The present invention relates to a bearing structure of a so-called hollow coreless motor.
2. Related Background Art
FIG. 6 shows an example of conventional hollow motor wherein an outer yoke 1a of a soft magnetic material serving as a stator and an inner yoke 1b constitutes the main body of a coreless motor with hollow double cylinders. On the external periphery of said inner yoke 1b there is fixed an annular magnet 2, and a coil 3 serving as a rotor opposed to the air gap portion said magnet 2 is provided rotatably with ball bearings 9, 10 with a suitable gap to said outer yoke 1a. A coil support member 4, supporting said coil 3, is provided with a commutator 4a and an output portion 4b for transmitting the rotation of the motor to the outside. On the other hand, a brush support member 5, composed of an insulating material such as plastics and fixed at an end of said outer yoke 1a, is provided with a brush support portion 6 for supporting brushes 7, which are in sliding contact with said commutator 4a and which receive power supply through lead wires 8 to rotate the rotor with said output portion 4b through already known commutating function.
Conventional hollow cylindrical motor, having an aperture along the center of rotation for example for passing a light beam or placing lenses therein is so constructed as to rotate through a screw coupling or a helicoid coupled as disclosed in the U.S. Pat. No. 4,152,060, and is associated with a loss in the transmission efficiency due to a friction in the rotary bearings. Also a lens-driving hollow cylindrical motor is disclosed in the U.S. Pat. No. 4,519,691, in which a movable coil 7 is fixed to a flange 10a provided at an outer end of a lens holder 10, and a bearing structure is composed of a ball bearing with a guide groove, provided at the flange.
In a hollow motor, the internal diameter of the hollow aperture at the center is limited by the dimensions of driving elements or other component parts, and such motor is still subjected to requirements for a smaller outer diameter and a shorter axial length. The use of the bearing known in the prior art not only results in an increase in the number of component parts, number of work steps for assembly and space required therefor. In addition, prior art bearings also poses limitations to the motor structure design. Consequently, it becomes necessary to develop a novel motor structure to accommodate the prior art bearing.
Also the aforementioned oil-containing sintered sliding bearing, in which lubrication is achieved by the oil flowing out from the bearing components to the sliding faces, may result in abnormal noises or abnormal abrasion due to mutual contact of metals at a lower temperature or at a low revolution where the amount of oil flow is lower.